The instant invention relates to means for sensing the acceleration profile of an object, such as a motor vehicle.
The prior art teaches acceleration sensors, or accelerometers, comprising a housing having an inertial or sensing mass within a cylindrical passage therein which is biased by suitable biasing means towards a first end of the passage. When the housing is subjected to an accelerating force which exceeds the threshold biasing force of the biasing means, the sensing mass moves along the passage away from the first end thereof toward a second position at the other end thereof, with such movement being retarded by suitable damping means therefor. Where the acceleration input is of sufficient magnitude and duration to displace the sensing mass to the second position within the passage, the sensing mass triggers switch means in the sensor, as by bridging a pair of electrical contacts therein, whereupon an instrumentality connected with the switch means, such as a vehicle passenger restraint system, is actuated. In this manner, the sensor mechanically integrates the acceleration input to the housing.
The prior art teaches a variety of spring and magnetic biasing means for biasing the sensing mass towards the first end of the passage and away from the switch means incorporated in the sensor. The use of springs, however, is discouraged due to the potential failure thereof due to breakage, whereupon the sensing mass is free to trigger the switch means when the housing is subjected to a de minimis accelerating force.
An accelerometer employing magnetic biasing means is taught in U.S. Pat. No. 4,329,549, issued May 11, 1982 to Breed, wherein a magnet secured to the housing proximate the first end of the passage exerts a magnetic biasing force on a magnetically permeable sensing mass, with the movement of the sensing mass being damped by a gas contained within the passage. Such magnetic biasing of the sensing mass offers the advantage of providing a maximum biasing force on the sensing mass when the sensing mass is in its initial position proximate the first end of the passage. However, as the sensing mass moves along the passage from its initial position therein towards the second position therein, the gas damping force quickly predominates in retarding the movement of the sensing mass. Thus, it will be readily appreciated that, upon the loss of the damping gas due to the failure of the seal which operates to maintain the gas within the passage, any acceleration exceeding the initial magnetic biasing threshold will cause the sensing mass to be fully displaced to the second position within the passage, thereby triggering the switch means of the sensor. In other words, an accelerometer constructed in accordance with the '549 patent is not able to properly mechanically integrate acceleration inputs thereto in the absence of the damping gas. Additionally, it is significant that the use of gas damping requires extreme tolerance control of the gap between the walls of the passage and the sensing mass thereof, thereby increasing manufacturing costs.
It is noted that electrical circuits incorporating the switch means of such known accelerometers typically employ a resistor to shunt the leads therefrom in order to provide circuit diagnosability. It will be readily appreciated, however, that circuit diagnostics are incapable of detecting the failure of such sensors in the "open" condition, e.g., when the sensing mass is unable to move to the second position within the passage, or when a change occurs in the damping characteristic of the sensor, as the shunting resistor will continue to indicate a fully functional sensor. Thus, the ability to test the operation of the sensor itself becomes critical to confirming the operability and, hence, reliability of a system incorporating such a sensor.